Resistance is the opposition to electric current caused by a material itself, and it stays the same regardless of frequency. Impedance is the total opposition to current in an AC circuit, combining resistance with the additional opposition created by capacitors and inductors. Both are measured in ohms, but impedance changes with frequency and includes information about timing shifts between voltage and current that resistance alone does not capture.
Resistance: Opposition That Stays Constant
Resistance is the simplest form of electrical opposition. When current flows through a resistor, some electrical energy converts to heat. That’s it. The resistor doesn’t care whether the current is steady (DC) or alternating (AC), and it doesn’t care how fast the AC signal oscillates. A 100-ohm resistor provides exactly 100 ohms of opposition whether the signal is 60 Hz, 10,000 Hz, or pure DC. This is the behavior described by Ohm’s Law: voltage equals current times resistance (V = IR).
Resistance is also a purely “real” quantity, meaning it can be expressed as a single number. There’s no timing shift involved. When voltage rises across a resistor, the current rises at the exact same moment. They stay perfectly in sync.
Impedance: The Bigger Picture in AC Circuits
Impedance is a broader concept that includes resistance but adds another layer called reactance. Reactance is the opposition to current flow created by two types of components: capacitors and inductors. Unlike resistors, these components store energy temporarily in electric or magnetic fields rather than converting it to heat. And critically, their opposition depends on the frequency of the AC signal passing through them.
A capacitor acts almost like an open circuit at low frequencies, blocking current flow. As frequency increases, its opposition drops and it begins to behave more like a short circuit, letting current pass freely. An inductor does the opposite. At low frequencies it acts like a plain wire with almost no opposition. At high frequencies it blocks current, behaving like an open circuit.
Impedance bundles resistance and reactance into a single value, represented by the letter Z. The generalized version of Ohm’s Law for AC circuits is V = IZ, where Z replaces R to account for the full picture of opposition in the circuit.
Why Impedance Needs Two Numbers
Here’s where things get interesting. Because capacitors and inductors shift the timing between voltage and current, you can’t describe impedance with a single number the way you can with resistance. In a circuit with an inductor, the voltage peaks before the current does. In a circuit with a capacitor, the current peaks first. This timing gap is called the phase angle, and it can be up to 90 degrees in either direction.
To capture both the amount of opposition and this timing shift, engineers express impedance as a complex number with a real part (the resistance) and an imaginary part (the reactance). If a circuit has a resistor, an inductor, and a capacitor all in series, the impedance combines all three contributions into one expression. The real part tells you how much energy the circuit dissipates as heat. The imaginary part tells you how much energy gets stored and released each cycle, along with the direction of the phase shift.
A purely resistive circuit has zero phase angle. Voltage and current peak together. Add a capacitor or inductor, and the phase angle shifts, meaning power delivery becomes less efficient because voltage and current are no longer perfectly synchronized.
Key Differences at a Glance
- Frequency dependence: Resistance stays the same at any frequency. Impedance changes as frequency changes, because the reactance contributed by capacitors and inductors is frequency-dependent.
- Energy behavior: Resistance only dissipates energy as heat. Impedance accounts for both energy dissipation and energy storage in electric and magnetic fields.
- Phase angle: Resistance has no phase angle; voltage and current stay in sync. Impedance carries a phase angle that describes how far voltage and current are shifted apart.
- Mathematical form: Resistance is a single real number (for example, 47 ohms). Impedance is a complex number with both a real and imaginary component.
- Circuit type: Resistance fully describes opposition in DC circuits or purely resistive AC circuits. Impedance is needed whenever capacitors or inductors are present in an AC circuit.
How Each One Is Measured
A standard multimeter measures DC resistance easily. It sends a small current through a component, measures the voltage drop, and calculates resistance from Ohm’s Law. This works perfectly for resistors and wire, but it tells you almost nothing about how a capacitor or inductor will behave in an AC circuit.
Measuring impedance requires a different instrument called an LCR meter. It applies a pure sine wave at a specific frequency to the component under test and simultaneously measures voltage, current, and the phase angle between them. Using that data, it separates the impedance into its resistive and reactive parts. This is why an LCR meter can detect problems that a multimeter misses. A capacitor might show the correct capacitance value on a multimeter but still fail in a real circuit because its resistive losses are too high, something only visible when you measure the full impedance.
Where This Matters in Everyday Life
Speaker and amplifier matching is one of the most common places people encounter impedance outside a textbook. Speakers are rated in ohms (typically 4, 6, or 8 ohms), but this number represents impedance, not simple resistance. It varies with the frequency of the audio signal. Matching your speaker’s impedance rating to your amplifier matters because a mismatch can cause distortion or even damage. Connecting a 4-ohm speaker to an amplifier designed for 8 ohms forces the amplifier to deliver more current than it’s built for, which can overheat or short-circuit it. Tube amplifiers are especially vulnerable to this kind of mismatch.
The same principle applies to headphones, where impedance ratings influence how loud they’ll play from a given source and how much detail you’ll hear. High-impedance headphones (around 250 ohms or more) need a dedicated amplifier to reach useful volume levels, while low-impedance models (under 32 ohms) work fine plugged directly into a phone.
Signal transmission is another area where impedance is critical. Coaxial cables, USB cables, and Ethernet cables are all designed with a specific characteristic impedance (commonly 50 or 75 ohms for coaxial). When the cable’s impedance doesn’t match the devices on either end, part of the signal reflects back instead of passing through cleanly. This causes data errors in digital systems and signal loss or ghosting in video systems. The cable itself has very little resistance, so resistance alone can’t explain or predict this behavior.
Resistance Is a Special Case of Impedance
The simplest way to remember the relationship: resistance is impedance with the reactive part set to zero. Every resistor has an impedance, and that impedance happens to be a real number with no phase shift. But not every impedance is a resistance. Whenever a circuit includes components that store energy (capacitors, inductors, or anything that behaves like them), you need the full concept of impedance to describe what’s happening. In DC circuits, where nothing oscillates, reactance drops out of the picture entirely and impedance collapses down to plain resistance. That’s why introductory electricity courses start with resistance. It’s the simpler, frequency-independent foundation that impedance builds on.